Various technologies have been developed in the past for stabilizing the discharge flow of molten steel from an immersion nozzle so as to produce a cast slab having excellent surface and internal quality. Japanese Patent Publication (A) No. 2002-301549 discloses a continuous casting method preventing the phenomenon of single-sided flow of molten steel in the casting mold by setting an angle between a sliding nozzle and horizontal plane formed by the discharge flow to 80 to 90°. Japanese Patent Publication (A) No. 58-74257 discloses an injection method making the immersion nozzle a rectangular cross-section and casting while holding the injection flow from the injection nozzle to the casting mold at a uniform low speed descending flow. Japanese Patent Publication (A) No. 9-285852 discloses a continuous casting method making the discharge hole a slit shape and dispersing and making uniform the flow of molten steel discharged from an immersion nozzle so as to produce a cast slab free from surface and internal defects.
Japanese Patent Publication (A) No. 2000-237852 discloses an immersion nozzle provided inside it with a twisted tape shaped rotating blade. Japanese Patent Publication (A) No. 9-225604 discloses a continuous casting method introducing inert gas into an immersion nozzle and controlling the internal pressure so as to prevent the occurrence of a biased flow in the flow of the molten steel from the discharge hole. Japanese Patent Publication (A) No. 9-108793 discloses a continuous casting method using an immersion nozzle with a front end enlarged in inside diameter compared with the inside diameter of the base end of the immersion nozzle.
However, even with these methods, it was still difficult to stabilize the flow of molten steel discharged into the casting mold. It was not possible to sufficiently prevent surface defects called “slivers” due to inclusions occurring at the coil surface after rolling or bubble defects called “blowholes” due to argon blown from the immersion nozzle.